Excited 2-hydroxybenzophenone

Hydroxybenzophenones represent the largest and most versatile class of ulbaviolet stabilizers that are used to protect materials from the degradative effects of ulbaviolet radiation. They function by absorbing ultraviolet radiation and by quenching elecbonically excited states. 

The apparatus used for picosecond flash spectroscopy on these systems has been described before(8 10). Figure 3a and b show typical transient absorption data obtained on 2-hydroxybenzophenone and the copolymer. Summary of these spectral data are given in Table 3. The transient observed at the shortest delay time (7ps) is the first excited singlet in all systems. The spectral data (at delay times > 50ps) permit placement of upper limits on triplet yields in CH2CI2 for both 2-hydroxy benzophenone itself and the copolymerized chromophore. 

The capability of 2-hydroxybenzophenone derivatives to dissipate light energy has been ascribed to rapid deactivation of the excited singlet state by intramolecular interaction between the carbonyl and hydroxyl groups, possibly involving reversible H-transfer. These proposals are outlined in Scheme I, where P and PP represent the polymer and photoproduct, respectively. 

We can illustrate the application of PAC to a simple photochemical reaction. Acetone is readily excited to its singlet excited state which rapidly undergoes efficient intersystem crossing to its triplet state. The triplet state decays in solution primarily by radiationless decay. The PAC experimental waveforms obtained from the photoexcitation of acetone in air and argon-saturated cyclohexane are shown in Fig. 1. In addition, the waveform obtained from the calibration compound 2-hydroxybenzophenone is also shown. 

Figure 1. Experimental waveforms from excitation (266 nm) of 2-hydroxybenzophenone (T-Wave, and acetone in air-saturated (E-Wave(l)) and argon-saturated (E-Wave(2)) cyclohexane. (With permission from Joshua L. Goodman.)...

It is well known that o-hydroxyphenyl ketones phosphoresce very weakly and do not undergo photoreduction363,368 or photoelimination,366 presumably because of rapid enolization in the excited state. Lamola has performed quenching studies which indicate lifetimes of o-hydroxybenzophenone triplets of less than 10"8 sec. However, these lifetimes, as well as phosphorescence efficiencies, are markedly enhanced by the addition of alcoholic solvents,383 presumably because hydrogen bonding slows down photoenolization. 

One thing is clear there can be no acid-base equilibrium between states of different multiplicities thus it is correct to consider only the pK of the singlet state , or the pA of the triplet state . However, the question of the protolytic equilibrium between an mr singlet and a tttt or charge transfer (CT) singlet remains open. This problem is illustrated in Figure 4.48 for the case of 4-hydroxybenzophenone, in which there is a reversal in the order of mr and CT states between the acid and base forms. Excitation of the protonated molecule in ethanol for example leads to the ground state deprotonated form, but the detailed mechanism of this process is not known. 

Commonly used ultraviolet absorbers such as hydroxybenzophenones, hydroxybenzotriazoles, and salicylic acid derivatives transfer absorbed light energy rapidly from their excited states, giving off thermal energy and longer wavelength light quanta (18). 

Several molecules which can undergo intramolecular proton transfer in the excited state have been found to be unusually photostable. Thus, for example, 2-hydroxybenzophenone is used as a photostabilizer in polymers while benzophenone itself is photoactive (Kysel, 1969). In crystalline 2-(2 -hydroxyphenyl)benzothiazole and its derivatives [3] a proton is transferred in the excited state from an... 

Some polymers show discoloration as well as reduction of the mechanical properties (e.g. aromatic polyesters, aromatic polyamides, polycarbonate, polyurethanes, poly (phenylene oxide, polysulphone), others show only a deterioration of the mechanical properties (polypropylene, cotton) or mainly yellowing (wool, poly(vinyl chloride)). This degradation may be less pronounced when an ultraviolet absorber is incorporated into the polymer. The role of the UV-absorbers (usually o-hydroxybenzophenones or o-hydroxyphenylbenzotriazoles) is to absorb the radiation in the 300-400 nm region and dissipate the energy in a manner harmless to the material to be protected. UV-protection of polymers can be well achieved by the use of additives (e.g. nickel chelates) that, by a transfer of excitation energy, are capable of quenching electronically excited states of impurities (e.g. carbonyl groups) present in the polymer (e.g. polypropylene). 

The role of u.v. absorbers such as the 2-hydroxybenzophenones and 2-hydroxybenzotriazoles have also attracted some interest. Ivanov and Anisimov believe that the former type stabilize by quenching photoactive excited species in polymers, whereas the latter type have been shown by Hodgeman and Gellert to be converted by hydroperoxides into active dienones. This latter... 

Scheme 9.8 Excited-state intramolecular proton transfer (ESIPT) in the case of 2-hydroxybenzophenone.

Some additives have more than one mechanism of action. In particular there are UV absorbers that are also quenchers. Cinnamates absorb in the range 290-320 nm and also act as quenchers, but they do not have good compatibility with polymers, and this reduces their effectiveness. Oxalanilides absorb light chiefly in the 280-340 nm range and can act as quenchers for carbonyl groups, protecting a wide range of polymers. 2-Hydroxybenzophenone and benzotriazole derivatives can neutralise free radicals, and can quench excited states in polystyrene. 

An overview of stabilizer mechanisms and t5q>es of chemicals used for each mechanism is provided in Table 3. Further descriptions of these mechanisms follow. The three major classes of UV absorbers are 2-hydroxybenzophenones, 2-hydroxyphenylbenzotriazoles, and the newer 2-hydroxyphenyl-s-triazines. The nickel phenolates not only have excited state quenching capability, but also provide radical scavenging and hydroperoxide decomposing activity. The radical scavengers include hindered benzoates and hindered amines. The hindered amines have the added capability to decompose hydroperoxides. Another class of... 

Substances satisfying these conditions are colorless and do not absorb in the high-wavelength range. It is possible to classify them according to their chemical structure as derivatives of 2-hydroxybenzophenone, esters of aromatic acids and aromatic alcohols, hydroxyphenyl-benzotriazoles, substituted acrylonitriles, metallic complexes (excited-state quenchers), and inorganic pigments [4,9,23,26,27]. 

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